The present invention relates to a control apparatus for controlling the displacement of a variable displacement compressor used in a refrigerant circuit of a vehicle air conditioner.
A typical variable displacement compressor includes a drive plate coupled to pistons. The drive plate is accommodated in a crank chamber. The pressure of the crank chamber is adjusted to alter the inclination angle of the drive plate, which varies the displacement of the compressor between the minimum displacement and the maximum displacement. The crank chamber pressure is adjusted by a control valve. Specifically, the opening degree of the control valve is adjusted based on a command from a controller.
Clutchless type compressors, which has no clutch, such as an electromagnetic clutch, between an external drive source, such as a vehicle engine, and the drive shaft of the compressor, have been introduced. Since a clutchless type a compressor has no electromagnetic clutch, which is expensive and heavy, the cost is lowered and the weight of the compressor is reduced. Also, since there is no shock due to engaging and disengaging of an electromagnetic clutch, the engine performance is improved.
A clutchless type compressor includes means for stopping external refrigerant circulation. The stopping means stops the refrigerant circulation through an external refrigerant circuit when the displacement of the compressor is minimized. When cooling is not needed, a controller provides a control valve with a signal for minimizing the compressor displacement. Accordingly, the refrigerant circulation through the external refrigerant circuit is stopped by the stopping means and unnecessary cooling is prevented. Also, the minimum compressor displacement allows refrigerant to circulate within the compressor through the discharge chamber, the crank chamber, the suction chamber and the compression chamber, which permits the sliding parts of the compressor to be lubricated by lubricant contained in the circulated refrigerant.
If the discharge pressure is excessive in the refrigeration circuit, the pipes of the circuit receives excessive load. Therefore, when a discharge pressure sensor detects a pressure that is higher than a predetermined level, the controller adjusts the command signal to the control valve such that the compressor displacement is gradually decreased until the discharge pressure falls below the predetermined level (for example, in Japanese Unexamined Patent Publication No. 59-112156).
However, an excessively increased discharge pressure may not be quickly lowered according to a decrease in the compressor displacement. In this case, the displacement may be dropped to a value that is close to the minimum displacement. Compared to a clutch type compressor, the minimum displacement of a clutchless type compressor is significantly small so that the power loss of an engine is minimized when refrigeration is not performed. Therefore, if the displacement is close to the minimum displacement, little refrigerant is supplied to the compressor from the external refrigerant circuit. That is, lubricant contained in the refrigerant is not sufficiently supplied to the compressor. Thus, the parts needing lubrication, such as sliding portions of the pistons and the cylinder bores, will be poorly lubricated.
Accordingly, it is an objective of the present invention to provide a control apparatus and a control method that reliably lubricate the sliding parts of a variable displacement compressor when lowering an excessive discharge pressure.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, an apparatus for controlling a variable displacement compressor used in a refrigerant circuit of an air conditioner is provided. The refrigerant circuit includes the compressor and an external circuit, which is connected to the compressor. The compressor compresses refrigerant sent from the external circuit and discharges the compressed refrigerant to the external circuit. The refrigerant circuit has a high pressure zone, which is exposed to the pressure of refrigerant that is compressed by the compressor. The compressor includes a tiltable drive plate, which is located in a crank chamber and changes its inclination angle in accordance with the pressure in the crank chamber. The inclination angle of the drive plate determines the displacement of the compressor. The apparatus includes a control valve, which adjusts the pressure in the crank chamber, and a controller for controlling the control valve. The controller sends a command value that corresponds to cooling performance required for the refrigerant circuit to the control valve. The control valve operates to adjust its opening according to the sent command value. When the pressure in the high pressure zone is equal to or higher than a predetermined threshold value, the controller sends a command value that can minimize the displacement of the compressor to the control valve thereby limiting the pressure in the high pressure zone.
Further, the present invention provides a method for controlling a variable displacement compressor used in a refrigerant circuit of an air conditioner. The refrigerant circuit includes the compressor and an external circuit, which is connected to the compressor. The compressor compresses refrigerant sent from the external circuit and discharges the compressed refrigerant to the external circuit. The refrigerant circuit has a high pressure zone, which is exposed to the pressure of refrigerant that is compressed by the compressor. The compressor includes a tiltable drive plate, which is located in a crank chamber. The drive plate changes its inclination angle in accordance with the pressure in the crank chamber. The inclination angle of the drive plate determines the displacement of the compressor. The method includes adjusting the pressure in the crank chamber by a control valve, wherein the control valve operates according to a command value, which represents cooling performance required for the refrigerant circuit, and sending a command value that can minimize the displacement of the compressor to the control valve thereby limiting the pressure in the high pressure zone when the pressure in the high pressure zone is equal to or higher than a predetermined threshold value.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.